With the development of radio and space technologies, several satellites based navigation systems (i.e. satellite positioning system or “SPS”) have already been built and more will be in use in the near future. SPS receivers, such as, for example, receivers using the Global Positioning System (“GPS”, also known as NAVSTAR, have become commonplace. Other examples of SPS systems include, but are not limited to, the United State (“U.S.”) Navy Navigation Satellite System (“NNSS”)(also known as TRANSIT), LORAN, Shoran, Decca, TACAN, NAVSTAR, the Russian counterpart to NAVSTAR known as the Global Navigation Satellite System (“GLONASS”) and any future Western European SPS such as the proposed “Galileo” program. As an example, the U.S. NAVSTAR GPS system is described in GPS Theory and Practice, Fifth ed., revised edition by Hofmann-Wellenhof, Lichtenegger and Collins, Springer-Verlag Wien New York, 2001, which is fully incorporated herein by reference.
The U.S. GPS system was built and is operated by the United States Department of Defense. The system uses twenty-four or more satellites orbiting the earth at an altitude of about 11,000 miles with a period of about twelve hours. These satellites are placed in six different orbits such that at any time a minimum of six satellites are visible at any location on the surface of the earth except in the polar region. Each satellite transmits a time and position signal referenced to an atomic clock. A typical GPS receiver locks onto this signal and extracts the data contained in it. Using signals from a sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time (i.e. navigation solution).
GPS and other satellite based navigational systems have some limitations such as the availability of a sufficient number of satellite signals. Satellite signals are sometimes not available in deep canyons, in areas with large number of buildings blocking the direct satellite signals, and in dense forest areas. In addition to this, the satellite signals can be completely blocked or greatly attenuated inside buildings. To reduce these errors, inertial measurement units (IMUs) equipped with microelectromechanical systems (MEMS) sensors can be integrated with a personal navigation device (PND) to provide data that is used to improve the position availability and reliability of the PND in degraded signal environments. For example, in an indoor environment where satellite signals are not available or a dense urban environment where multipath errors are common, MEMS sensor data can aid in the calculation of a navigation solution. IMUs include gyroscopes that measure changes in direction, accelerometers that estimate acceleration, magnetic sensors that can detect changes in the orientation of a device, and a host of other similar devices.
More particularly, after the position of a PND is initially determined, the IMUs allow the position of the PND to be determined as the PND moves, even if the satellite signals are blocked. The determination of a position by propagating a previous known position based on movement data (e.g., data provided by an IMU) is known as dead reckoning (DR), or inertial navigation. Currently, DR methods do not take into account how the PND is moving other than detecting changes in velocity, acceleration or heading.